Natural gas is distributed to residential, commercial and industrial users through underground gas mains. When natural gas service is required at a previously unserviced premises, a service line is connected to the gas main to divert a flow of gas to the premises.
Self tapping service tees provide a fast and economical means of connecting the service line to a steel gas main. A self tapping service tee comprises a hollow steel body for connection to the main, having a threaded interior wall engaging a perforator and a service outlet projecting from the body for connection to the service line.
One end of the service tee is welded to the main and the service line is connected to the service outlet. The perforator, which has a solid body provided with a threaded exterior surface complimentary to the interior threading of the tee body and a cutting tip for piercing the main, is threaded into the body through an opening at the free end. The perforator is rotated within the body by a ratchet wrench, and recedes into the body until the cutting tip pierces the wall of the main and removes a small disc-shaped section of the main wall, known as a "coupon". At this point the perforator blocks the flow of gas out of the main. The perforator is then rotated in the other direction and retracted to the free end of the service tee body to admit gas into the tee body and through the service outlet. The perforator thus serves as a temporary plug to prevent gas from escaping during the connection procedure, and also retains the pipe coupon. Sealing compound is applied to exterior threading about the free end of the body and to the threading of a cap, which is screwed onto the free end of the tee body to complete the connection.
One of the most common operations performed by natural gas utilities is the removal of service tees from a gas main, known as a "pin-off" procedure.
According to the conventional procedure for removing a service tee, the steel cap is removed from the free end of the tee and the perforator is removed from the tee body. With gas escaping at high pressure from the free end of the tee, a steel rod is measured and marked to ensure sufficient penetration into the main. The rod is inserted into the free end of the service tee body, forced through the body against the pressure of the escaping gas and hammered into the opening in the main until a complete seal is obtained. The rod is measured again to ensure proper penetration into the main. The gas is blown down at the service line, and the service tee is cut with a hacksaw between the service outlet and the main. The detached portion of the service tee is lifted up and over the rod and discarded. The rod is welded to the main and soap tested for leaks. Once the seal is established to be gas-tight the rod is cut as close to the main as possible, and a coating of mastic is applied to the repair before backfilling the excavation.
This procedure presents potential hazards due to the danger inherent in having gas escaping from the service tee at high pressure during much of the procedure. Moreover, an element of human error is introduced by reliance upon measurements of the rod to ensure proper penetration into the main.
In another procedure the steel cap is removed to expose the opening in the free end of the body. The perforator is rotated back into the body until its frustoconical seat plugs the opening in the main, and the gas is blown down at the service line. In this position the perforator is well beyond the service outlet. The tee body is cut between the service outlet and the main, usually through the solid body of the perforator, and the detached portion of the service tee is discarded. The remaining piece of the perforator is welded to the remains of the service tee body (the "pin-off"), the weld is cleaned with a grinder, and a steel cap is welded on top of the pin-off to prevent mechanical damage. The welds are soap tested for leaks before the excavation is backfilled.
In this procedure the perforator becomes a permanent plug, the reliability of which can be suspect if the perforator seat does not precisely seal the opening in the main. During installation the cutting tip of the perforator cuts straight through the main wall, so the frustoconical perforator seat contacts the main only about the exterior edge of the opening, and the slow cutting action of the perforator does not usually leave a smooth edge around the hole. Thus, the welding step becomes particularly important to ensuring that a gas-tight seal is maintained.
In either of these pin-off procedures, in order to perform the procedure a sufficiently large area around the service connection must be excavated to enable a technician to work on the main from within the excavation. Although excavation is generally not itself a costly process, the repair of such an excavation, particularly where the service connection is beneath a road or sidewalk, can be extremely costly.
Further, there are always safety concerns associated with having service personnel working inside an excavation next to a live gas main.